Variable band amplifier



Oct. 8, 1929. F. K. VREELAND 1,730,937

VARIABLE BAND AMPLIFIER Filed July 20, 1926 3 Shqets-$heet l Get. 8, 1929. F. K. vRE ELAND VARIABLE BAND AMPLIFIER Filed July 20, 1926 5 Sheets-Sheet 2 Oct. 8, 1929. F. K. VREELAND 1,730,937

VARIABLE BAND AMPLIFIER Filed July 20. 1926 3 Shoots-Sheet 3 ll'll-ll' lllllllllll'lll'll lllllllll'll' u n n n u m fiviamoa @M Patented 0a. 8, 1929 "UNITED STATES PATENT I oF c FREDERICK K. VBEEI-AND, OF MONTCLAIB, JERSEY, ASSIGNOR TO VBEELA ND CORPORATION, 01' NEW YORK, N. Y., A CORPORATION OF NEW JERSEY VARIABLE BAND AK PLIFIER Application filed July 20,

The invention herein described relates to amplifiers of alternating currents and receiving apparatus inc ding such amplifiers. It relates particularl' to a band amplifier with continuously adjustable band, and the com bination of such an amplifier with a separate frequency selector.

The general purpose of the invention is to produce an amplifier and receiving apparatus of improved efiiciency, effectiveness and simplicity of manipulation. Its particular object is to secure a high degree of amplification over a band of frequencies of any desired width, whose position in the frequency scale may be determined at will by a simple adjustment, while the amplification at frequencies outside of such band is so small that interference at such frequencies is practically eliminated. A further object isto secure a high degree of selectivity without a corresponding distortion of the signal wave by narrowing the frequency band of effective amplification.

Other desirable features of the invention are explained at length herein.

.In my application for U. S. Patent Serial No. 655,791 I have described an amplifier embodying certain novel features and possessing high efiicicncy, freedom from disturbing effects by stray fields, either electrostatic or magnetic, and an amplification-frequency characteristic which may be determined at will to secure a variety of desirable qualities. I have also described a specific embodiment of the broader features of the invention in a multi-stage amplifier in which the amplification-frequency characteristics of the several stages are so chosen and so associated with each other that the over-all amplification-frequency characteristic of the amplifier is a band which, in the preferred form, has a high and substantially uniform amplification over a certain frequency range and a relatively very small amplification at frequencies outsidethis range. I have also de- 1926. Serial No. 123,619.

scribed in that application, and claimed in a divisional application, filed April 14, 1925, Serial No. 22,928, means for shifting the band in the frequency scale or coordinate by adding successive increments of reactance, preferably capacity, simultaneously to the coupling devices or transformers of the several stages, thereby altering at will, by a single operation, the range or band of frequencies over which the amplifier is effective while maintaining its general band characterstic.

In my applications for U. S. Patents Serial Nos. 582,603, 582,604, 680,061 and 680,062 I have described a tuner or frequency selector of novel character having high eflicienc and selectivity combined with great simplicity of manipulation, and havin characteristics that render it peculiarly a aptable for use in combination with a band amplifier in carrying out the present invention.

The invention herein described utilizes certain of the desirable features of the inventions set forth in my applications referred to, together with other and novel features Whereby further desirable results are secured.

In the form of the invention specifically described and claimed in my application Serial No. 22,928, the reactances, or specifically the capacities, added to the several stages of the amplifier to shift the band, are units having suitable relative values predetermined to give each band the desired form and frequency range, these reactances being added simultaneously by a single control means. In the present invention I prefer to employ capacities which are added by continuous increments or gradations and so are continuously and simultaneously variable by a single control means, so that the position of the band may be adjusted and placed atany desired point in the frequency sc'ale. These capacities are so related to each other and to the other amplifier constants that the form of the band may be predetermined, and its width and amplification chosen to suit the desired requirements. In the preferred form of'the invention the band is progressively and automatically narrowed as the working frequency s lowered, and the overall amplification is maintained substantially constant over the entire frequency range of the amplifier. Another feature of the invention is means for controlling the width of the band and the overall amplification while retaining the other desirable features, including constant amplification over the entire range of frequency control.

Another feature of the invention is the combination of a band amplifier with a tuner or frequency selector whereby very high selectivity is secured without distortion of the wave form of the received signals. When it is sought, as usual in the art heretofore, to secure high selectivity, or the power to separate desired signals from strong interfering-signa-ls,'by sharpness of tuning, or to increase selectivity by operating tuned amplifiers or other tuned circuits, tuned to the same or signal frequency, in cascade, the increase in selectivity results in sharpening and narrowing the peak of the frequency characteristic of the system. When this increase of selectivity is carried to the point where powerful interference can be eliminated the frequency characteristic of the system has such a narrow peak that the side bands of a modulated wave are materially cut off with resulting distortion of the articulation or tone quality. By the use of the present in vention these difiiculties are avoided and I am able to secure very high selectivity without narrowing the signal wave band.

Other features of the invention are set forth in the following:

In the drawing, Figure 1 shows a simple embodiment of the invention including a typical amplifier incombination with a two-circuit tuner and antenna.

Figure 2 shows an alternative arrangement of the amplifier combined with a single circuit tuner employing a loop collector.

Figure 3 shows a detail of construction of one form of condenser suitable for use in the arrangements of Figures 1 and 2.

Figure shows the individual and overall characteristics of the amplifier under different conditions.

Figure 5 shows curves illustrating other characteristics of the apparatus.

Referring to Figure 1, A ,'A A D are the' several amplifier tubes of a multi-stage amplifier, the last named tube, D,-being employed as a detector.

In the arrangement shown, T, T, T are coupling coils or transformers comprising primary windings p, p, p and secondary wind ings s, s, s, the outer terminal a of each primary being connected to the plate or anode of the preceding amplifier tubeand the outer the last named connection including a suitable bias when desired, as shown. C C C are bypass condensers bridging these inner terminals, which are preferably employed to shunt the high frequency currents around the batteries.

The preferred type of coupling T shown in this and the succeeding figures is a transformer having its primary and. secondar windings wound in opposite directions, sac of these windings made in two mutually re versed sections, thus embodying the feature of astaticism orabsence of stray field, utilizing the mutual capacity between the coils (indicated by the dotted capacity C) to assist the magnetic coupling of the coils, and possessing the other desirable features fully set forth in my applications herein referred to. This type of transformer is particularly desirable because of its high efiiciency and the absence or minimizing of stray :fields tending to produce regeneration. It is not necessary, however, to employ this type of transformer or coupling means, as many of the desirable features of the invention may be obtained by the use of other well known types of coupling, including those in which special devices are employed to neutralize the effect of undesirable feed-back couplings when these are present. These special devices are not required when my preferred coupling means are used.

8, 8, 8 are band-shifting capacities associated with the coupling devices or transformers which, in the specific arrangement shown in this figure, are variable condensers connected across the outer terminals a, b of the transformers T, T, T respectively, so that their capacities are added to the internal or inherent capacities of the several transformers. These condensers are associated by a mechanical connection, for example, by connecting their axes or shafts 11, 11, 11 together by couplings 12, 12 which are preferably insulated, so that their capacities may be simultaneously varied by a single operation such as rotating the condensers on the common axis by means of a control handle 9. The characteristics of the three stages of the amplifier are thus simultaneously varied. The condensers are preferably of similar construction so that their capacities are varied by equal increments. The total capacities of these condensers ma be equal at all oints though they are pre erably constructed with a constant'capacity difference, represented by an additional or spacing capacity 10, as will be explained at length hereinafter. The transformers T and their associated capacities 8 and 10 together constitute the coupling means linkin .nected to a second tuning capacity 0 The first amplifier tube A is connected in shunt across the terminals of the coil L and condenser C A stabilizing resistance B may be included in this connection. C, is a fixed compensating capacity in parallel with the condenser G and C is a variable adjusting capacity, for adjusting the compensation to the particular antenna employed, in the manner fully explained in my tuner application, above mentioned.

The capacities C and G are mechanically connected with the capacities 8, 8, 8, of the amplifier, so that all are manipulated by a single control means. The capacities of the tuner and the amplifier are designed to be varied by similar increments, and the inductance of tuner and amplifier are also so chosen in relation to these capacities, that the frequency characteristic of the tuner is thesame as that of the amplifier, so that the frequency for which the tuning system is tuned will always lie within the band of effective amplification of the amplifier. If desired, the capacity C may be made adjustable for the purpose of synchronizing the tuner and amplifier, but I find that this is unnecessary when the parts of the apparatus are properly designed, particular y when the band of the amplifier is made wide enough to include any slight variations in the tuning system, as I.

prefer.

The correct design to insure synchronizing of the tuner circuits and the amplifier band at all frequencies is not difficult to obtain. For example, if all the band-shifting capacities 8 are made variable by equal increments, and the tuner capacities C, and C are also made variable by equal increments, having a suitable ratio to the capacity increments of the band-shifting capacities, then by making the overall effective inductance of the transformers or other coupling coils bear the inverse ratio to the effectiveinductance of the tuner coils L L,, the frequency curve of the tuner with respect to condenser position will have substantially the same form as that of the amplifienband. These curves are readily brought into coincidence by a suitable choice of capacity of the compensating and adjusting capacities, Cx and C or by a mutual rotational adjustment of the tuner and amplifier shafts 11 through the appropriate coupling 12. Suitable modifications of this general relation to suit particular cases will be readily understood by the designing engineer.

The coupling means, including the transformers T, T, T, are so constructed, in the preferred form of the apparatus, that the amplification-frequency characteristics of the several stages are different; for example, they may. have such characteristics as are represented by the curves 1, 2, 3, in group A j of Figure 4. The overall characteristic of the amphfier is determined by the combination of these individual characteristics. If the vforms of the individual characteristics are suitably related to their spacing in the frequency scale, the overall characteristic will have the form of a substantiallyflat band as shown by curve 4 of group A in Figure 4. The means whereby this is accomplished in the preferred form of transformer here shown is fully explained in my former applieations herein referred to. The difference in the individual characteristics of the coupling means may be obtained by various means, such as a difference in the windings of the several transformers, or a difference in the inherent mutual capacity between the windings, or the use of the additional or spacing capacities 10. In the last named case the individual transformers may be all alike or equal in their mechanical construction and the desired characteristics obtained in the manner setfbrth at length in the following:

Whenthe capacities of the band control condensers 8, 8, 8 are changed, as by rotating the common axis 11 by means of the control handle 12, the individual characteristics of the several stages are altered. An increase of capacity of the control condensers will shiftthe individual characteristics of the stages toward lower frequencies, and the combined or overall band characteristic will also be shifted toward a lower frequency band, as shown in curves 4 of groups B and G of Figure 4. Since the condensers 8, 8, 8 are continuously variable, the overall band characteristic is variable by continuous gradations,

that is, it may be located at will at any point within the frequency range for which the apparatus is designed by the single operation of moving the control handle 9. With this variation, however, the band retains its characteristic form. In the arrangement that I prefer to employ, the apparatus gives substantially constant amplification over the frequency range of the band with a very sharp cut-off at frequencies above and below the band, as ilustrated in the curves 4; of Figure 4.

Figure 2 shows a modification in which the band control condensers 8, 8, 8 instead of bridging the outer terminals a, b of the transformers T are shunted across the terminals of the secondary coils s, s, s respectively. Such a shunted capacity has the efiect of shifting the characteristics of the several the simultaneous variation of the several condensers the overall band characteristic is shifted in a manner similar to that explained in reference to Figure 1. In this Figure 2 the selectiveinput system shown is a single circuit tuner with loop collector.

The condensers 8, 8, 8 instead of being shunted across the secondary coils s, s, 8, may be shunted across the primary coils p, p as fully explained and illustrated in my ormer applications referred to, or otherwise associated with the coupling means.

In any of the arrangements shown and described above the desired individual characteristics of the several stages of amplification may be obtained by differences in the construction of the transformers, for example, by different numbers of turns in the primary or secondary windings or both, or by altering the distance between the windings or the area of the juxtaposed surfaces, thus altering the internal mutual capacity between the windings, or by other means, as explained in my former applications 655,794 and 22,928. It is not necessary, however, to make the several transformers or coupling devices of different construction. The transformers may be, and preferably are, made of identical construction, thus securing simplicity in the mechanical design and manufacture, and the spacing of the individual characteristics of the several stages requisite to secure the desired band characteristic may be obtained in the following manner, shown schematically in Figures 1 and 2, and illustrated in detail in Figure 3.

The band-shifting condensers 8, 8, 8 are preferably made alike, or at least variable by equal increments, and the desired spacing of the characteristics of the several stages in the frequency scale is obtained by adding fixed spacing capacities 10, 10 in one or more of the stages, so that the total capacities differ from each other by constant'amounts as the band-shifting capacities are varied. Preferably the spacing capacities 10, 10 have difi'erent values so that each stage of the amplifier has a different characteristic. These spacing capacities may be separate elements or condensers shunted across the variable condensers, or they may be incorporated in the same structures as the variable or band-shifting capacities 8. A simple form of mechanical construction is shown in Figure 3, in which the spacing capacity is a fixed plate 10 placed in juxtaposition with one or more of the other fixedplates 13 of the condensers but connected to the movable plates 14 as shown. In the prefered form of the apparatus the fixed plates 13 and movable plates 14 of the condensers are of the usual well known construction and are alike in all the condensers 8, and the spacing capacity 10 is given the value required to produce the desired differstruction of the band-shifting capacities 8 and spacing capacities 10 may be employed in the bridging position shown in Figure 1, or in shunt across the secondary coils s, as shown in Figure 2, or across the primary coils,

. as has been explained.

The arrangement employing identical coupling transformers and different total capacities in the several stages not only possesses the advantages of mechanical simplicity and ease of construction that have been explained, but it also possesses distinct electrical advantages as will be understood by reference to the curves in Figures 4 and 5. In Figure 5 the ordinates represent values of the variable or band-shifting capacities 8 and the abscissae represent the corresponding peak values of the individual characteristics of the amplifier. Figure 4c shows three typical groups of such characteristics. Thus the curve 1 of Figure 5 represents the relation between the band-shifting capacity and the peak frequency or frequency of greatest amplification corresponding to the curves 1 of groups A, B, and C in Figure 4. Similarly curve 2 of Figure 5 represents the variation of peak frequency with band-shifting capacity corresponding to the curves 2 of groups A, B and G. Curve 3 similarly shows the relation of peak frequency to capacity for the curves 3 of Figure 4. The three curves of Figure 5 are similar in form but are progressively spaced in the capacity coordinate, that is, for given values of the frequency abscissae, the coresponding values of the capacity ordinates of the curves have constant differences, d, d, d and e, e, 6 respectively. The constant differences d and d+e correspond to the constant spacing capacities, 10, 10 of Figures 1 and 2, which remain fixed as the band-shifting capacities are equally varied.

Thus, the curve 3 of Figure 5 may represent the effect of increasing values of the band-shifting capacity 8 on the peak frequency of a given transformer without an added spacing capacity, such as the third stage of Figure 1 or Figure 2. Curve 2 represents this efiect when a constant spacing capacity 10, equal to d, is added to the capacity 8, as in the second stage of Figure 1 or Figure 2. Curve 3 represents this effect when a constant spacing capacity equal to (1+6 is added, as in the first stage, the bandshifting capacities 8 being always equal for the three stages.

Because of the curvature of these curves and their constant spacing in the capacity 00- ordinate, their spacing in the frequency coordinate will vary, being greater as the capacity is less and the frequency greater. Thus it is seen that the curves come closer together, in the frequency scale, capacity is added to decrease the frequefiey. This change in the spacin of thetransformer peaks is illustrated in l igure 5 for three values of the band-shifting capacity A, B and C, and the coresponding change of the transformer characteristics is shown by the dotted lines connecting the peaks of curves 1, 2 and 3 of Figure 4 with the corresponding points on curves 1, 2 and 3 of Figure 5.

There is an important relatioirbetween the capacity-frequency curves of Flgure 5 and the corresponding am lification-frequencv characteristic curves of igure 4. In general, I have found that when the frequency is lowered by adding capacity to any one of the amplifier stages the maximum amplification of thatstage is usually also lowered and its characteristic curve is narrowed. Figure 4 shows this change of form, somewhat exaggerated. The curves 1, 2, 3, of group A represent the characteristics of the several stages with a small value of the band-shifting capacity 8. Curves 1, 2, 3 of group B show the individual characteristics with a larger value of the band-shifting capacity, and curves 1, 2, 3 of group C showv the characteristics with a still larger value of the capacity. The sev= eral curves of group A, group B and group C have successively diminishing values of peak amplification and successively narrowing characteristics, as shown in the figure.

But it will be noted that the curves come closer together as capacity is added to de-' crease the frequency. By suitably choosing the various constants in the design of the apparatus this squeezing together of the peaks may be carried to such a degree that the increased mutual effect of the several stages completely offsets the falling off of amplification at the lower frequencies, so that the overall band characteristic has constant amplitude and the amplification of the device is constant over the entire range, as shown by the o'verall band curves 4 of groups A, B and C. l The corresponding narrowing of the band is' more apparent than real since the effective width of the band in the frequency scale should be considered in relation to the frequency. Thus the band width, measured in kilocycles, should preferably be less at lower frequencies in proportion to the frequency, as it is in fact.

By suitably choosing the values of the spacing capacities 10, the band may be given any desired width within reasonable limits, determined by the characteristic of the particular transformer or coupling device employed. To this end the capacities 10 may be made adjustable, if desired.

The above described characteristics of the band-amplifier render it particularly useful as an element ofa receiving system including a tuner or frequency selector, preferably of the type set forth in my applications referred to. The present day crowdin of the available radio channels makes the problem of avoiding interference increasingly difiicult; Ordinary methods of tuning,,including the use of tuned circuits or sharply tuned amplifiers 1n cascade, when carried to the point where adequate selectivity is obtained, are ob- JQCtlOIltlblG, because the characteristic curve of a sharply tuned system is essentially peaked, and cuits in cascade narrows the overall curve as a whole and increases the shar ness of the peak. lVhen this process is carried to the extent of effective selectivity the peak of the frequency characteristic becomes so narrow that the side bands of a modulated wave are excluded or weakened to a serious extent and the resulting signal is distorted.

By the use of the present invention the highest selectivity may be secured without distortion.

To this end I prefer to design the band amplifier so that its range of effective amplification is sufficient to include the entire side bands of the modulated wave, and also sufficient to permit some latitude of adjustment,

thus avoiding the need of extreme mechanical precision in the apparatus. With this amplifier I prefer to combine a-tuner or selective circuit designed with a frequency characteristic just sufiiciently broad to include the de:- sired side bands of modulation. Thisis preferably a two circuit tuner with single control of the type described in my applications above mentioned and illustrated in Figure 1 of the present application. This supplies the preliminary tuning of the system while the band amplifier, having a sharp cut-off on both sides the multiplication of tuned cirof the band, efi'ectually eliminates powerful interference outside the band. This combination, besides being highly efi'edtive as a selective system, permits great simplicityin adjustment. -The tuner control is mechanically connected to the amplifier control, as shown in the drawings, and bdth'are operated by a single motion of the control wheel.

Such unit control with high selectivity is rendered practically feasible by the fact that the band amplifier permits the usual and reasonable variations in mechanical con struction while still securing effective al'n plification and frequency cut-off, and the tuner furnishes the fine adjustment.-

A further advantage of. the two circuit tuner which I have described as an element of this system is the loose coupling between its two circuits, which not only permits ready adjustment of the signal strength but is largely effective in eliminating local disturbances or ground noise that are troublesome when the antenna or collector is directly associated with the amplifier system,

' pose of increasing the selectivity by The utility and effectiveness of the devices herein described will be readily apparent by comparison with other devices that are usual in the art. Any receiving system that depends for its selectivity on tuning or resonance alone is ineffective for'securing high selectivity combined with distortionless reception of a modulated wave, for the reason that a modulated wave contains not only the fundamental or carrier frequency, which is often loosely referred to as the frequency of the wave, but includes also side bands comprising frequencies differing from this frequency by plus or minus the various frequencies of the voice of other modulation source. To receive a modulated signal Without distortion requires the reception of all these component frequenciesin their proper proportions.

When selectivity is accomplished by resonance alone, either simple resonance or the multiple resonance in a plurality of circuits operated in cascade and tuned to the signal frequency, it is impossible to increase selectivity beyond a certain rather low degree without so narrowing the peak of the resonance curve that a part of the side bands is trimmed off. This degree of selectivity is not sufiicient to meet the conditions of powerful interference existing in the art today. Consequently any such system based on simple or multiple resonance or tuning will fall short of the desired selectivity or will distort the reception of the modulated wave.

By the use of the present invention I am able to obtain the highest selectivity and eliminate completely the most powerful local interference without distortion of the received wave, in the manner already explained.

Referring to the arrangement of Figure 1, the receiving circuit L (3,, with its associated antenna, is tuned to the point of efficient reception of the signal wave, but not sharply enough to modify the strength of any of the frequencies included in the side bands. The

second tuned circuit L 0 is similarly tuned.

to the point of efficiency without distortion. These two circuits are used, not for the purmultiple tuning with consequent distortion, ut chiefly for the sake of the loose coupling which permits an efficient intensity control and eliminates disturbance due to electric currents in the neighborhood or other ground noise.

The current wave received by this second circuit, including all the side bands, is then applied to the band amplifier, which amplifies all its frequencies alike without distortion. This is accomplishes in the following manner :The first stage of the amplifier, whose characteristic has a peak value above or below the wave frequency, amplifies all the frequencies of the transmitted wave, but not all alike, because of the slope of its characteristic. The second stage, whose characteristic has a different peak frequency, amplifies all the 'wave frequencies still further, but in 'a different proportion because of the different slope of its characteristic. The third stage again amplifies still further all the frequencies of the transmitted wave but again in different degrees corresponding to its different slope. By means of the relation between these characteristics explained herein, any frequency whose amplification is deficient in the first stage of the amplifier is amplified in greater degree in one of the other stages, and vice versa, with the result that, for every frequency within the effective band, the overall amplification of the three stages combined is substantially the same, as is shown clearly in the curves of Figure 4.

In other words, for any given frequency within the effective band, if the characteristic curve of one stage of the amplifier has a downward slope the characteristic of one or more of the other stages will have a corresponding upward slope, and these slopes are so related to each other in the preferred form of the invention that these opposite slopes neutralize each other, producing a uniform over-all amplification curve.

It will be noted also from Figure 4 that, at frequencies above or below a certain value, all the characteristic curves of the several stages have a steep 'slope the same way, with the result that, the effective amplification drops off sharply to a very small or negligible value. Thus i terfering signals of such frequencies are not amplified to any appreciable extent and are eliminated from the receiver..

band may be varied within wide limits so that the highest selectivity may be secured with distortionless reception for a signal wave band of any desired width.

When the adjustment of the apparatus is varied for different signal frequencies the tuned receiving circuit or circuits and the several stages of the amplifier are all varied in similar degree, retaining their mutual relations, so that the amplifier becomes effective for a new band of frequencies which always includes the frequencies transmitted by the receiver circuits. By the device of squeezing together the characteristics of the several amplifier stages as the frequency is lowered an extremely uniform degree of amplification is secured over a wide range of frequency adjustment, and a very common difficulty existing in systems based: on multiple resonance is cured, namely, the serious falling off of amplification when the frequency passes a certain point. 7

It will be understood that the invention is not limited to the particular forms of constituction shown but is applicable to a variety of constructions.

What I claim is: IL The method of receiving a signal wave which consists in performing substantially uniform amplification over a definitely limited band of frequencies and varying the position of this band in the frequenc scale y continuous gradations to include t e frequencies of the signal Wave. 2. Theinethod of selectively receiving a signal wave which consists in performing effective amplification over a definite band of frequencies, effecting a sharp cut-off in the amplification at frequencies outside this band,

and varying by continuous gradations the position of the band of effective amplification 1n the frequency scale to include the frequencies of the desired signal wave and exclude undesired frequencies.

3. The method of selectively receiving a signal Wave which consists in effecting preliminary selection of the signal frequencies, performing effective amplification over a definitely limited band of frequencies, including the selected signal frequencies andexcluding undesired frequencies, and simultaneously varying by continuous gradations the resinance of the preliminary selection and the position of the frequency band of amplification to include these frequencies.

4. The method of receiving a signal wave which consists in performin successive stages of amplification having difierent frequency characteristics and in combination producing effective amplification over a band of frequencies, and varying simultaneously by continuous graduations the frequency characteristics of the several stages and the resulting position of the band of effective amplification in the frequency scale, to include the frequencies of the signal Wave.

5. In a receiving system for alternating currents, an amplifier giving substantially uniform amplification over a definite band of frequencies, and means for varying by continuous gradations the position of this band in the frequency scale While maintaining its hand character.

6. In a receiving system for alternating currents, an amplifier giving effective amplification over a definite band of frequencies with a sharp cut-off for frequencies outside this band, and means for varying by continuo'us gradations the position of this band in the frequency scale While maintaining its band character and sharp cut-off.

7. In a receiving system for alternating currents, an amplifier giving effective amplification over a band of frequencies, means for varying by continuous gradations the position of this band in the frequency scale and simultaneously narrowing the band as the frequency is lowered, thereby maintaining subcuit, an amplifier giving effective amp stantially constant amplification over the entire'range of frequency variation.

8. In a radio receiving system, the combination of a broadly selective receivin circuit, an amplifier giving effective amp cation over a band of frequencies, and means for varying the frequencies passed b the selective circuit and simultaneously shifting the efi'ectiveband of the amplifier to include this frequency.

9. In a radio receiving system, the combination of a broadly selective receiving fpircation over a band of frequencies, and means for varying by continuous gradations the frequencies passed by the selective circuit and simultaneously shifting the effective band of the amIplifier to include this frequency.

10 n nation of a selective receiving circuit with associated collecting means, means for compensating the reactance of the collecting means, an amplifier iving effective amplification over a band of frequencies, and means for varying by continuous gradations the frequency of the receiving circuit with its associated collecting means and simultaneously shifting the effective band of the amplifier to include this fre uency. x a

11. In an amplifier of alternating currents, a plurality of successive amplifying units having different frequency characteristics and in combination giving effective amplification over a band of frequencies, and means for varying by continuous gradations the fre quency characteristics of the successive units and thereby shifting the position of the resulting band of amplification in the frequency scale While maintaining its hand character.

12. In an amplifier of alternating currents, a plurality of suecesive amplifying units having different frequency characteristics spaced with substantial uniformity in the frequency scale and in combination givingeffective amplification over a band of frequencies, and means for varying by continuous gradations the frequency characteristics of the successive units While maintaining their substantially uniform spacing and thereby shifting the position of the resulting band of amplification in the frequency scale.

13. In an amplifier of alternating currents, a plurality of successive amplifying units having different frequency characteristics and in combinationgiving effective amplification over a band of frequencies, and means for shifting the characteristics of the successive amplifying units in the frequency scale by continuous gradations and simultaneously varying their proximity in the frequency scale, thereby shifting the position of the resulting band of effective amplification in the frequency scale, While maintaining its hand character and securing substantially uniform a radio receiving system, the combiquency adjustment.

capacity 14. In an amplifier of alternating currents, a plurality of amplifying units having different frequency characteristics and in combination giving efi'ective amplification -over a band of frequencies, and means for adding simultaneously to the several amplifying units by continuous increments, thereby eral units and shifting the band of effective amplification in the frequency scale while maintaining its band character.

15. In an amplifier of alternating currents, a plurality of amplifying units each comprising coupling means, including capacities that are similar except in their capacity values,

and means for simultaneously and continuously varying these capacities by equal increments with a constant capacity difference, whereby the frequency characteristics of the several units are simultaneously and continuously but differently varied.

16. In an amplifier of alternating currents, a plurality of amplifying units each comprising a coupling coil with associated variable capacity, the amplifying units with their associated. capacities having different frequenc characteristics and in combination giving e ective amplification 'over a band of frequencies, and means for simultaneously and continuously varying the associated capacities, whereby the band of effective amplification is shifted in the frequency scale while maintaining its band character.

17. In an amplifier of alternating currents, a plurality of amplifying units having equal coupling coils with associated variable capacity, and means for simultaneously and continuously varying the associated capacities by equal increments with constant capacity difference, and thereby producing effective amplification over a continuously variable band of frequencies.

18. In an amplifier of alternating currents, a plurality of amplifying units, comprising equal coupling transformers, capacities continuously variable by equal incremements associated with each transformer, and additional spacing reactances associated with the coupling transformers whereby the characteristics of the several units are spaced in the frequency scale.

19. In an-amplifier of alternating currents, a plurality ofainplifying tubes, a coupling coil associated with each tube, a continuously variable band shifting capacity associated with each coupling coil, additional spacing reactances associated with thecoils whereby the characteristics of the several units are spaced in the frequency scale, and

means for simultaneously and continuously varyin the band shifting capacity.

20. n a radioreceiving system, a selective receiving circuit including an-adjusting cavarying the characteristics of the sevincludin band s ifting capacity and means i for simu taneously and continuously varying the capacities of the tuned receiving circuit and of the amplifier by increments so related to each other and to their associated parts that the fre uency of the receiving circuit is varied and t e frequency band of the amplifier is simultaneously shifted to include the frequencies passed by the receiving circuit.

21. In a radio receiving system, a selective receiving circuit with associated collecting means and including an adjusting capacity, means for compensating the reactance of the collecting means, a continuously variable band amplifier receiving si al energy therefrom and including band s ifting capacities, and means for simultaneously and continuously varying the capacities of the tuned receiving circuit and the band amplifier by increments so related to each other and to the associated parts that the frequency of the receiving circuit is varied and the frequency band of the amplifier is simultaneously shifted to include the frequencies passed by the receiving circuit.

22. In a radio receiving system, a selective receiving circuit with associated collecting means and including an adjusting capacity, a second receiving circuit coupled thereto and including a second adjusting capacity, means for compensating the reactance of the collecting means and securing similarity between the two receiving circuits, a continuously varying band amplifier receiving signal ener from the second circuit and including and shifting capacities, means for simultaneously and continuously varying the capacities of the two receiving circuits and of the band amplifier by incrementsso related to each other and to their associated arts that the frequencies of the two receivmg circuits are equally varied and the frequency band of the amplifier is simultaneously shifted to include the frequency of the tuned receiving circuit.

23. In an amplifier of alternating currents, a plurality of amplifying tubes, coupling coils connecting the amplifying tubes in cascade, capacities associated with the several coupling coils, means for varying these capacspacing reactances effective amplification over a band of frequencies, and means for simultaneously and continuously varying the band shifting capacities whereby the position of the band of effective amplification in the frequency scale is continuously varied.

25. In an amplifier of alternating currents, a plurality of amplifying tubes, coupling coils connecting the amplifying tubes in cascade, spacing reaetances separating the frequency characteristics of the coupling coils, band shifting capacities variable by equal increments and giving in combination with the coupling coils and the spacing reactances effective amplification over a band of frequencies, and means for simultaneously and continuously varying the band shifting capacities whereby the position of the band of effective amplification in the frequency scale is continuously varied.

26. The method of amplifying high frequency currents which consists in performin successive stages of amplification so relate to each other as to produce in combination effective amplification over a band of frequencies with a sharp cutsoif at frequencies outside this band, and adding reactances to the several stages by continuous increments so related to the other constants of the system as to shift continuously the band of effective amplification in the frequency scale while preserving its hand character and cut-off.

27. The method of amplifying high fre quency currents which consists in performing successive stages of amplification having different frequenc characteristics, spacing these yCllZLI'flCtQIlStlCS 1n the frequency scale with an overlap so related to the form of the characteristics as to reduce in combination effective amplification over a band of frequencies with a sharp cut-off for frequencies outside this band, and shifting the several characteristics in the frequency scale by continuous increments so related to each other as to preserve the efi'ective spacing and produce in combination a band of effective amplification continuously shifted in the frequency scale wiile preserving its band character and cut- 28. In an amplifier of high frequency currents, a plurality of amplifier tubes, coupling means connecting the tubes in cascade so related to each other as to roduce in combination effective amplification over a definite band of frequencies with a sharp cut-off, and means for adding reactances to the several stages by continuous increments so related to the reactances of the coupling means as to shift the band ofefi'ective'amplification continuously in the frequency scale while preserving its band character and cut-off.

29. In an amplifier of high frequency cur-1 rents, a plurality of amplifier tubes with associated coupling means having different fre:

quency characteristics, means for spacing these characteristics in the frequency scale with an overlap so related to the form of the 

